1) Field of the Invention
The present invention relates to a mobile station apparatus and a transmission power control method for the same apparatus and, for example, it relates to a technique suitable for use in a system employing an HSDPA (High Speed Downlink Packet Access) transmission mode which is one of transmission modes for mobile communication systems.
2) Description of the Related Art
In 3GPP (3rd Generation Partnership Project), there has been standardized a W-CDMA (Wideband-Code Division Multiple Access) mode which is one mode of the third generation mobile communication system. In addition, the HSDPA which provides a transmission rate of a maximum of approximately 14 Mbps in a downlink is provided as one subject matter of the standardization.
The HSDPA employs an adaptive coding modulation mode and is characterized by making the switching between, for example, the QPSK modulation mode and the 16-value QAM mode according to a radio environment between a base station and a mobile station apparatus (which will hereinafter be referred to equally as a “mobile station or mobile unit”).
As the main radio channels to be used in the HSDPA, there are HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel) and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
Each of the HS-SCCH and the HS-PDSCH is a common channel in a down direction (i.e., a direction from a base station to a mobile station, and the HS-SCCH is a control channel for transmitting various types of parameters related to data to be transmitted through the HS-PDSCH. Among the various types of parameters, for example, there are modulation type information indicative of which of modulation modes is used for transmitting data through the HS-PDSCH and information such as the number of spread codes to be allocated (the number of codes) and a pattern of rate matching to be conducted with respect to transmission data.
On the other hand, the HS-DPCCH is an individual control channel in an up direction which is an direction from a mobile station to a base station, and this HS-DPCCH is used in a case in which a mobile station transmits an ACK signal and an NACK signal to a base station according to the propriety of reception of data received through the HS-PDSCH. In a case in which the mobile station fails to receive the data, for example, when the received data shows a CRC (Cyclic Redundancy Check) error, the NACK signal is transmitted from the mobile station, and the base station carries out re-transmission control.
In addition, the HS-DPCCH is also used in a case in which a mobile station, which has measured a reception quality (for example, SIR: Signal to Interference Ratio) of a received signal from a base station, transmits a result of the measurement as a CQI (Channel Quality Indicator) to the base station. On the basis of the received CQI, the base station makes a decision on the quality of radio environment in a down direction and, if the quality is high, makes the switching to a modulation mode which can transmit data at a higher speed and, if the quality is low, makes the switching to a modulation mode for transmitting data at a lower speed (that is, adaptive modulation).                “Channel Structure”        
Secondly, a description will be given hereinbelow of a channel structure.
FIG. 7 is an illustration of a channel structure in the HSDPA. The W-CDMA employs the code division multiplex, and each channel is separated by a code.
First of all, a brief description will be given hereinbelow of channels which are not mentioned yet.
Each of CPICH (Common Pilot Channel) and SCH (Synchronization Channel) is a common channel in a down direction.
The CPICH is a channel to be utilized for channel estimation in a mobile station and cell search, and as a timing reference for the other down-direction physical channels in the same cell, so-called a channel for the transmission of a pilot signal. The SCH is exactly classified into P-SCH (Primary SCH) and S-SCH (Secondary SCH), and is a channel for a burst-like transmission with 256-chip at the head of each slot. The SCH is received by a mobile station made to carry out three-stage cell search, and is used for establishing slot synchronization and frame synchronization.
Likewise, with reference to FIG. 7, a description will be given hereinbelow of the relationship between timings of a channel.
As shown in FIG. 7, in each channel, 15 slots (each slot corresponds to a 256-chip length) constitute one frame. Since the CPICH is used as a reference for other channels as mentioned above, the head of a frame of each of the SCH and the HS-SCCH coincides with the head of a frame of the CPICH.
In this case, although the head of the frame of the HS-PDSCH is delayed by two slots with respect to the HS-SCCH and others, this is for, after a mobile station receives modulation method information through the HS-SCCH, enabling the demodulation of the HS-PDSCH according to a demodulation method corresponding to the received modulation method. Moreover, in the HS-SCCH and the HS-PDSCH, three slots constitute one subframe.
The HS-PDSCH is a channel in an up direction, and the first slot thereof is used for the transmission of an ACK/NACK signal indicative of a result of reception of the HS-PDSCH from a mobile station to a base station after the elapse of approximately 7.5 slots from the reception of the HS-PDSCH. Moreover, the second and third slots thereof are used for the periodic feedback transmission of CQI information for adaptive modulation control to a base station. In this case, the CQI information to be transmitted is calculated on the basis of a reception environment (for example, a result of SIR measurement of CPICH) measured for a period of time between the last-but-three slots and the last slot with respect to the CQI transmission.
The ACK or NACK signal for the notification of the propriety of reception of the HS-PDSCH can also be issued repeatedly two or more times according to setting.
That is, a mobile station, which has received a notification on an announcement of transmission of the HS-PDSCH through the first subframe A of the HS-SCCH, demodulates and decodes the HS-PDSCH (first subframe E) delayed by two slots to carry out a CRC check for detecting the presence or absence of an error.
If the decision shows no error, an ACK signal (slot C in FIG. 7) is transmitted through the use of the first slot of the subframe delayed by approximately 7.5 slots from the reception of the HS-PDSCH and the same ACK signal (slot D in FIG. 7) is further transmitted repeatedly through the use of the first slot after one subframe. Naturally, in the case of the detection of an error, the NACK signal is repeatedly transmitted.
In this case, although it is also possible to inhibit the repeated transmission of the reception result, when the same ACK signal or NACK signal is repeated transmitted N times (N represents a natural number), the ACK signal or NACK signal is more reliably received by a base station, thereby avoiding the useless re-transmission control.
However, since the ACK signal or NACK signal is repeatedly transmitted by the next subframe, it is inhibited to transmit the HS-PDSCH to the same mobile station by the succeeding N subframes including the next subframe.
This is for preventing the occurrence of no discrimination between the ACK signal (slot D in FIG. 7) related to the repeated transmission of the reception result (ACK signal, NACK signal) on the first subframe E of the HS-PDSCH corresponding to the first subframe A of the HS-SCCH and the ACK signal related to the first transmission of the reception result (ACK signal, NACK signal) on the second subframe F of the HS-PDSCH corresponding to the second subframe B of the HS-SCCH.
Furthermore, a communication system using an HSDPA transmission method has a transmit power control (TPC) function to solve the so-called “far and near problem” by controlling the transmission power between a base station and a mobile station.
The “far and near problem” signifies the following phenomenon.
In general, with respect to a radio electric wave, the attenuation quantity increases as the transmission distance becomes longer. For example, in the case of a communication system based on the CDMA mode, since the frequency bands to be used by a plurality of mobile stations (users) exist mixedly in the same frequency band, in a case in which the transmission power of a signal transmitted by a mobile station close to a base station is high, the transmitted signals from the other mobile stations positioned at a greater distance with respect to the aforesaid mobile station suffer the interference with the transmitted signal from the mobile station close thereto, which affects the transmission/reception between the mobile stations and the base station.
For this reason, for preventing the “far and near problem”, for example, a communication system based on the CDMA mode always monitors the communication quality and uses TPC command bits (sometimes, which will hereinafter be referred to simply as a “TPC command” or “TPC bit”), thus carrying out the TPC for, if the communication quality is low, increasing the transmission power of a faraway mobile station and, if the communication quality reaches a sufficient level and the transmission power is excessive, decreasing the transmission power of the mobile station.
FIG. 8 shows a frame format for the TPC bits. In FIG. 8, the TPC bits in a down direction (downlink, i.e., direction from a base station to a mobile station), together with TFCI (Transport Format Combination Indicator) bits for displaying a format of a transport channel, are mapped in a control channel (DPCCH: Dedicated Physical Control Channel) of a DPCH (Dedicated Physical Channel) which forms an individual physical channel in the downlink, and are time-multiplexed with a DPDCH (Dedicated Physical Data Channel), forming a data channel, within a slot having 2560-chip time per slot. Accordingly, 1 slot=2560 chip time becomes a cycle of a high-speed TPC.
In addition, one frame is composed of 15 slots (slots #0 to #14), and this frame is continuously transmitted to a mobile station on a cycle of 10 ms.
FIG. 9 shows values to be taken by the DPDCH and the DPCCH. As shown in FIG. 9, 49 formats are provided with respect to one slot, and each defines a channel bit rate, a channel symbol rate and others, while one of four numbers, i.e., 2, 4, 8 and 16, is taken with respect to the number of TPC bits (NTPC).
Furthermore, FIG. 10 is an illustration of a conventional configuration for the TPC processing in a base station and a mobile station. In FIG. 10, 200-1 and 200-2 represent base stations (Node-B#1, Node-B#2), and 201 designates a mobile station (UE: User Equipment). In this illustration, although the base stations are two in number, naturally, the configuration also applies to a case in which three or more base stations exist. In the case of no discrimination between the base stations 200-1 and 200-2, the base stations are designated simply at 200.
Each of the base stations 200-1 and 200-2 shown in FIG. 10 is made up of, when taking note of an essential part thereof, a matched filter 202, a RAKE combiner 203, an SIR measuring unit (SIR measurement after side diversity) 204, a channel decoder 205, a block error rate (BLER) measuring unit (BLER measurement) 206, adders 207 and 209, a target BLER memory 208, a target BLER calculating unit (Target SIR) 210 and a TPC command generator 211.
In the base station 200-1 (200-2) thus configured, an up-direction (uplink, i.e., direction from a mobile station to a base station) signal transmitted from the mobile station 201 is inputted through a reception antenna (not shown) to the matched filter 202 and, after subjected to the inverse spread processing in this matched filter 202, it is inputted to the RAKE combiner 203 and the SIR measuring unit 204.
The RAKE combiner 203 conducts the RAKE combination processing on the inputted signal, and the channel decoder 205 conducts the channel decoding processing on the combined signal. Moreover, the BLER measuring unit 206 measures of the BLER of the received signal on the basis of the decoded data, and the adder 207 detects a difference between the measured BLER and a target BLER stored in the target BLER memory 208, and the target BLER calculating unit 210 calculates a target SIR on the basis of the detected difference.
Meanwhile, the SIR measuring unit 204 measures an SIR on the basis of an inputted signal from the matched filter 202, and the adder 209 detects a difference between the measurement result and the target SIR obtained by the target SIR measuring unit 210. Moreover, the TPC command generator 211 generates a TPC command on the basis of the detection result, with the generated TPC command being transmitted to the mobile station 201 through the DPCCH of the DPCH in the downlink as mentioned above.
On the other hand, the mobile station 201 shown in FIG. 10 is, for example, made up of, when taking note of an essential part thereof, TPC processing units 201-1 and 201-2 corresponding to the base stations 200-1 and 200-2, an adder 221, a channel decoder 222 and a TPC command combiner 225. Each of the TPC processing units 201-1 and 201-2 is composed of a matched filter 212 for DPCH, a matched filter 213 for CPICH, channel estimating units (channel estimation) 214 and 215 for each reception antenna (Antenna 1, Antenna 2) (not shown), multipliers 216 and 217, RAKE combiners 218 and 219, an adder 220, a TPC symbol soft decision unit (TPC symbol Soft decision) 223 and a TPC command decision (hard decision) unit (TPC Command Decision (Hard Decision)) 224.
In the mobile station 201 having this configuration, signals transmitted from a plurality of base stations 200-1 and 200-2 are inputted through reception antennas (not shown) to the matched filters 212 and 213 and subjected to the inverse spread processing using channelization code of the DPCH or the CPICH so that a DPCH/CPICH signal is separated and extracted, with the received signal of the DPCH being inputted to the channel estimating units 214, 215 and the multipliers 216, 217.
In addition, each of the channel estimating units 214 and 215 carries out a required correlative operation on a received signal of the DPCH inputted from the matched filter 212 on the basis of a received signal (pilot signal) of the CPICH inputted from the matched filter 213 so as to obtain an channel estimate for each antenna (Antenna 1, Antenna 2) with respect to the base station 200-1, 200-2, and each of the multipliers 216 and 217 multiplies this channel estimate by the received signal of the DPCH obtained by the DPCH matched filter 212 for carrying out the channel compensation processing.
The received signals after the channel compensation are further inputted to the RAKE combiners 218 and 219 to be subjected to the RAKE combination processing and are added (combined) in the adder 220 and, subsequently, in the adder 221, further added (combined) to the signal (i.e., received signal from the other base station) obtained in like manner by the other TPC processing unit 201-2 and then channel-decoded in the channel decoder 222, thereby providing received data.
Still additionally, the combined signal obtained by the adder 220 is inputted to the TPC symbol soft decision unit 223 and, after subjected to the soft decision processing in the TPC symbol soft decision unit 223, it undergoes the hard decision processing in the TPC command decision unit 224.
In this case, as the TPC bits to be mapped in the DPCCH, a plurality of bits (one number of 2, 4, 8 and 16) are taken according to a slot format (see FIG. 9) and, hence, in the TPC symbol soft decision unit 223, a slot format decision is made (step S200 in FIG. 14), and a soft decision combination is made with respect to a plurality of TPC symbols (steps S201 and S202 in FIG. 14), and a TPC command from each radio link (RL) (base station 200-1, 200-2), obtained in this way, is subjected to the hard decision combination in TPC command decision unit 224 (step S203 in FIG. 14).
In consequence, if it is larger than 0, the TPC command decision unit 224 determines 1 (increases the transmission power (UP)) and, if smaller than 0, the TPC command decision unit 224 determines 0 (decreases the transmission power (DOWN)) (steps S203 and S204 in FIG. 14).
The TPC commands from the respective RLs, detected by the TPC processing units 201-1 and 201-2 as mentioned above, are combined in the TPC command combiner 225 and, in a case in which all the TPC commands received from all the RLs (RL#i (i=1 ton) in FIG. 14) stand at 1 (increasing the transmission power), the control for increasing the transmission power is executed (outputting 1) while, in other cases (if at least one of the TPC commands received from all the RLs stands at 0), the (DOWN) control for decreasing the transmission power is executed (outputting 0) (steps S205 and S206 in FIG. 14).                Soft Decision Combination and Hard Decision Combination        
As mentioned above, the TPC bits are mapped in the DPCCH and stand at one of 2 bits, 4 bits, 8 bits and 16 bits according to a slot format. When the transmission is made by wireless, since the DPCCH is I- and Q-mapped, the following description will be given of, for example, a case in which the TPC bits are 4 bits in the QPSK.
In a case in which the base station 200 transmits the TPC bits (4 bits) “1111 (UP)”, since 2-bits transmission is made with 1-symbol in the QPSK, the data of (1, 1) is transmitted with 2-symbol (I, Q) as shown in FIG. 11(a).
In the case of the soft decision combination, instead of a decision based upon only 0/1, the TPC bits are decided in an analog fashion.
For example, let it be assumed that 1-symbol I, Q and 2-symbol I, Q are (0.5, 0.8, −0.3, 0.5).
At this time, the reception side I-Q constellation becomes as shown in FIG. 11(b). The soft decision combination is made by decomposing it into the I component and the Q component and obtaining the sum total of the respective bits.
In the case of the soft decision combination of the 4 bits, the sum total of the respective bits is obtained by the following equation (1).0.5+0.8−0.3+0.5=1.5  (1)
Since the sum total thus obtained exceeds 0 (in this case, 1.5), the TPC command (TPC_cmd) is set at 1, and a decision is made that it is a power increasing (UP) request.
On the other hand, in the case of the hard decision combination, since a decision is made on the basis of higher or lower values than the zero references of the I and Q components, the aforesaid TPC bits become (1, 1, 0, 1).
That is, in the case of the hard decision combination, the decision on a signal is made on the basis of only 1 or 0 while in the case of the soft decision combination, the decision is made on the basis of not only 0/1 but also information indicative of the degree of reliability.
In a conventional TPC processing mode, as mentioned above, the soft decision combination on the TPC bits is made within one RL and the hard decision combination is made on a result of the soft decision combination, thus determining which of UP/DOWN this RL notifies. Moreover, for the combination among a plurality of RLs, on the basis of the TPC commands from the respective RLs, if at least one RL indicates DOWN, the mobile station 201 operates so as to decrease the transmission power.
The non-patent document 1, mentioned later, discloses an algorithm related to a combination method for TPC commands transmitted from a plurality of base stations.TPC_cmd=(W1, W2, . . . WN) where TPC_cmd can take the values 1 or −1  (2)
According to this algorithm, the TPC commands from the plurality of base stations 200 are processed on the same condition (no weighting) and are decided equally. Moreover, according to the non-patent document 2, mentioned later, as shown in FIG. 12, the test condition is prescribed and a rule exists with respect to the accuracy at the TPC command combination of different RLs (between the base station 200 and a cell).
On the basis of these specifications, when the TPC commands received from all the RLs stand at 1 (increasing the transmission power), the mobile station 201 executes the control for increasing the transmission power and, in other cases (if at least one of the TPC commands received from all the RLs stands at 0), carries out the control for decreasing the transmission power. In other words, the control is executed so as to reduce the transmission power in a case in which there is a difference among the TPC commands received from a plurality of RLs.
That is, each of W1, W2, . . . WN (N represents a natural number) in the aforesaid equation (2) signifies a result of the soft decision combination of the TPC commands from the RLs corresponding to 1 to N and, when the TPC command (TPC_cmd) indicates 0 or 1 with no correlation, with respect to TPC_cmd from all the RLs, the value of the function γ is set at 1 in a case in which the rate of TPC_cmd standing at 1 exceeds ½N while it is set at 0 in a case in which the rate of TPC_cmd standing at 0 exceeds 0.5 (½). Moreover, as a rule, TPC_cmd becomes 1 (enhancement of power) when all the TPC commands from a plurality of RLs stand at 1 (increasing the transmission power) and it becomes −1 (reduction of power) when at lest one of the TPC commands from the RLs stands at 0 (decreasing the transmission power).
For example, as shown in FIG. 13(a), in “Test 1” (test case 1) in FIG. 12, under a static environment which does not have interference or fading, the TPC command from a cell 1 shows (0, 0, 1, 1) and the TPC command from a cell 2 shows (0, 1, 0, 1) and, hence, the control for an increase of power is executed only when both the TPC commands stand at 1 and the control for a decrease of power is executed in other cases.
Moreover, as shown in FIG. 13(b), in “Test 2” (test case 2) in FIG. 12, under a multipass fading environment, a rule is set with respect to the TPC commands from all the RLs so that the power is increased when the rate that the TPC command indicates 1 exceeds 0.25 (25%) while the power is decreased when the rate that the TPC command indicates 0 exceeds 0.5 (50%).
The patent document 1, mentioned later, discloses that, at the occurrence of a re-transmission request from a mobile station, a signal on the re-transmission request is transmitted through a radio channel whereby the transmission power from a base station reaches a predetermined value while satisfying a necessary reception quality in that mobile station and, hence, when the transmission power from the base station is set at the aforesaid predetermined value, the transmission power from the base station is suppressible to a minimum, thereby reducing the interference with the other mobile stations which do not receive the signal on the aforesaid re-transmission request.
In addition, the patent document 2, mentioned later, discloses that increasing and decreasing quantities of transmission power levels in a mobile station and in a base station are determined on the basis of the weighting coupling of a plurality of factors (at least three of route, number of base stations, position, power control command and ON/OFF of interference cancellation) representative of the present channel situation or the value of the previous power control command.
Still additionally, the patent document 3, mentioned later, discloses that, when a directional antenna is applied to an AAA (Adaptive Array Antenna) system, a transmitting/receiving mobile station is selected so that the directional beams addressed to the respective mobile stations do not interfere with each other and, for the purpose of allocating a radio resource to the selected mobile station, the SIR or the like is utilized with respect to the quality information received by the mobile station for determining the directional beam.
[Patent Document 1] Japanese Patent Laid-Open No. 2003-78480
[Patent Document 2] Published Japanese translation of a PCT Application, No. 2002-537712
[Patent Document 3] Japanese Patent Laid-Open No. 2003-235072
[Non-Patent Document 1] 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer procedures (FDD) (Release 6) (3GPP TS25.214 V6.7.1 (2005-12))
[Non-Patent Document 2] 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; User Equipment (UE) radio transmission and reception (FDD) (Release 7) (3GPP TS 25.101 V7.2.0 (2005-12))
According to the above-described conventional techniques, although all the TPC commands from a plurality of base stations are compared equally under the same condition and one TPC result is derived finally, the TPC command obtained from a far away base station has a high error rate in the process thereof and, hence, a reception result in a mobile station can be recognized in error. That is, a mistaken recognition on the power control information from a faraway base station can occur (the transmission power UP is recognized in error as the transmission power DOWN, or vice versa).
FIG. 15 is an illustrative view showing one example of the relationship in position between a mobile station and a base station. In FIG. 15, for example, the base stations 200 are three (200-1, 200-2 and 200-3) in number, and FIG. 16 shows the relationship among TPC commands transmitted from these base stations 200-1 to 200-3, TPC commands received on the mobile station 201 side and transmission power control in this case. The following description will be given of an example of a case in which the distance between the base station 200-3 and the mobile station 201 is longer than the distance between the other base station 200-1 or 200-2 and the mobile station 201 and the reception power of the TPC command from the base station 200-3 falls into a dropped condition.
At this time, although the original TPC commands to be transmitted from the base stations 200-1, 200-2 and 200-3 are in the UP state (increasing the transmission power), since the distance between the base station 200-3 and the mobile station 201 is long, a misjudgment on the TPC command from the base station 200-3 occurs in the mobile station 201 due to bit error (the TPC command from the base station 200-3 is recognized as DOWN), thereby leading to the transmission power control (DOWN) different from the transmission power control (UP) expected originally.
Thus, in a case in which a TPC command from a faraway base station, which has a reliability generally lower than that of the other base station, is handled in the same way (without weighting) as a TPC command from the other base station having a reliability higher than that of the aforesaid faraway base station, there is a possibility of power control in error for the above-mentioned reasons. That is, due to the above-mentioned mistaken judgment, the interference with the other cells can occur when the transmission power form a mobile station increases excessively. Moreover, when the transmission power from a mobile station further decreases than needed, difficulty is experienced in transmitting an up signal to a base station (which is referred to as “out of up synchronization”).
Therefore, as mentioned above, from the viewpoint of radio network, when the transmission power in a mobile station is controlled to a minimum in a range allowing the reception by a base station and satisfying a desired error rate (Target BLER), it is possible to suppress the wasteful use and contribute to an increase in number of users to be multiplexed.